1. Technical Field
The present invention relates in general to data storage system controllers for data processing systems, and more particularly to a method and system for managing serial communication between a storage system controller and a plurality of host computer systems. Still more particularly, the present invention relates to a method and system for eliminating the retransmission of unnecessary messages within a minimum time from a storage system controller to one of a plurality of host computer systems.
2. Description of the Related Art
In large data processing systems, it is common to utilize specialized hardware and software to control input/output devices. An example of one such specialized device is the "3990 Model 3 Storage Control" available from International Business Machines Corporation. The 3990 is a storage system controller which, in part, off-loads host-processor workload by receiving input/output commands from the host computer system, translates these commands to the Direct Access Storage Device (DASD), and manages the transfer of data between the host computer and the DASD. In performing these functions, the storage system controller transmits messages, such as "requests for connection" or "Request Ins," to a plurality of host computers, each connected to the storage system controller via a communications link or logical channel.
Depending on the particular application, communications hardware and software within the storage system controller may control transmission of messages over multiple independent logical channels connecting the storage system controller to multiple host computers. The communication protocol of such logical channels is defined by the data processing system interconnection architecture, an example of which is the "IBM Enterprise Systems Connection" (ESCON).
Under ESCON architectural requirements, a message, such as a Request In, transmitted from a storage system controller to a host computer, requesting connection to a logical channel, cannot be retransmitted for at least 50 microseconds. To prevent the storage system controller from generating a message representing a request for connection (for example, in a system utilizing the 3990, a "Request In") to the same host computer within 50 microseconds of a previous request for connection, a timing device monitors and prohibits retransmission of such a message from each individual logical channel for a specified period of time, which, in this example, is 50 microseconds. Such a timing and communication control device may be implemented in either hardware or software.
In known storage system controllers having multiple serial channels, each channel utilizes a separate timing device for prohibiting the transmission of a specific message within a specified period of time. Consequently, each serial channel employs substantially similar hardware or software to implement such a timer and control function. If the timing device is implemented in hardware, the use of redundant timer circuits raises manufacturing cost, consumes more power, generates more heat, and requires more space on a circuit board. Thus, it should be obvious to one skilled in the art that a need exist for a method or system of enhancing the efficiency of communication between multiple host computers and a storage system controller by prohibiting a serial channel from generating a request for connection within a specified period of time from a previous request for connection, without utilizing redundant timer circuitry and thereby using fewer parts, consuming less power, generating less heat, and requiring less circuit board space.